The invention described below was made in part with government support. The United States government has certain rights in the invention.
As well-known to those of ordinary skill, a standard real-time two-dimensional (2D) ultrasound scan typically entails the following. Referring to FIG. 1, an operator holds a transducer 100 in one position relative to a volume of material 102, e.g., human tissue. The transducer 100 is sometimes referred to as a scanhead; it commonly has an essentially linear, one-dimensional (1D) shape, although scanheads of round or other shapes are also known, and emits a beam of ultrasound energy toward the material 102 within a "scan plane" 103. The ultrasound energy is reflected from the material 102 and detected by the scanhead, which generates data signals representative of the detected energy. A conventional ultrasound machine 105 receives and processes the resulting data from the scanhead 100 and displays a 2D image of the tissue volume 102 being scanned, e.g., on a video display terminal 107, a film camera, or other hard copy device (not shown). Movement of the scanhead 100 results in different 2D views of the tissue volume 102 being presented.
Three-Dimensional (3D) data can be derived from a series of 2D views taken from different angles or positions. These views are sometimes referred to as "slices" of the actual three-dimensional tissue volume 102; the data sets used to generate these views are referred to here as "data slices." Experienced radiologists and similarly trained personnel can often mentally correlate a series of 2D images derived from these data slices to obtain useful 3D information. For example, a radiologist observing a scan of 2D views of a pregnant woman's abdomen may be able to diagnose the fetus's cleft palate by repeatedly moving the scanhead 100 and mentally correlating the 2D images presented.
Automated 3D image reconstruction has been done in the past by (a) using mechanical or other means of encoding the successive positions of the scanhead 100 as it is moved about the tissue volume 102, and (b) processing this additional encoded data to provide an indication of the relative positions of the 2D images acquired. This is often difficult to do on a real-time basis. Several technical problems have restricted the use of such systems, including noise interference and limits on spatial resolution. A related approach is to force the scanhead 100 to move over a predetermined track, but that can be adversely affected by movement of the material 102, which often happens in scanning human bodies.
Efforts are already being made by some ultrasound scanner manufacturers and others to produce specialized 2D hardware for 3D imaging. Such 2D hardware can be expensive to produce and cumbersome to use, with problems including the current cost and size of the scanhead, image artifacts produced by the fluid path used, etc. Replacing the current 1D arrays with full 2D arrays for making true 3D images is not yet practical due to the number of elements that would be required in the transducer, connections to these and the electronic channels required to services the additional elements. The ultrasound industry is presently interested in the potential for 3D imaging technology which would not obviate the current cost advantage of 1D scanheads.
Speckle Tracking of Moving Tissues
The invention described and claimed below makes a novel use of image analysis, or analysis of RF (radio frequency) data used to create ultrasound images, to indicate the relative position of such images in a three dimensional data set. For example, in one implementation, a speckle correlation technique is used that is well-known in other contexts: When a source of coherent energy (e.g., a laser or an ultrasound beam) interacts with a collection of scatterers, the amplitude of the reflected signal varies in relation to the relative positions of the scatterers. Displayed visually, this amplitude variation appears as a speckle pattern.
Ultrasound speckle correlation techniques have been used to determine the movement of blood. Consider an example: Suppose that the material 102 is a human blood vessel with blood cells moving through it. The blood cells are scatterers of the ultrasound energy generated by the scanhead 100, meaning that the amplitudes of the reflected signals vary in relation to the relative positions of the blood cells.
The blood cells are continuously circulating in the body, however. Consequently, specific blood cells are continuously moving out from under the scanhead 100 (and out of the ultrasound energy beam) and being replaced by others.
As a result, the speckle pattern of the reflected signal is continuously changing because of the movement of the blood cells. This is referred to as "decorrelation" of the signal with respect to the original speckle pattern. By monitoring the amount of decorrelation using standard techniques, the rate at which blood moves under the scanhead may be monitored.
The invention described and claimed below was developed in the course of efforts to create 3D images of the breast, to improve the quality of breast cancer diagnosis. It proved difficult to obtain 3D ultrasound images of breasts using conventional mechanical registration of scanhead position. The difficulty was particularly acute for small, dense breasts, which are well recognized as also presenting problems for X-ray mammography diagnosis.